原题链接在这里：

题目：

Design an algorithm to encode a list of strings to a string. The encoded string is then sent over the network and is decoded back to the original list of strings.

Machine 1 (sender) has the function:

string encode(vector
    
      strs) {  // ... your code  return encoded_string;}
    

Machine 2 (receiver) has the function:

vector
    
      decode(string s) {  //... your code  return strs;} 
    

So Machine 1 does:

string encoded_string = encode(strs);

and Machine 2 does:

vector
    
      strs2 = decode(encoded_string);
    

strs2 in Machine 2 should be the same as strs in Machine 1.

Implement the encode and decode methods.

题解：

encode 时维护一个stringbuilder, 对于每一个string, sb append 该string长度 + "/" + string内容.

decode 时开始index = 0, while index < s.length(), 利用indexOf("/", fromIndex)找index 后面第一个 "/"的index, index 与 "/"index 之间是长度，解析后面该长度的string, 更新index到"/"index + 1 + len.

Time Complexity: encode, O(n). decode, O(n). n是strs list的所有string包含所有char的个数.

Space: O(n). StringBuilder sb 大小.

AC Java:

1 public class Codec { 2  3     // Encodes a list of strings to a single string. 4     public String encode(List
     
       strs) { 5         if(strs == null || strs.size() == 0){ 6             return ""; 7         } 8         StringBuilder sb = new StringBuilder(); 9         for(String s : strs){10             int len = s.length();11             sb.append(len).append("/");12             sb.append(s);13         }14         return sb.toString();15     }16 17     // Decodes a single string to a list of strings.18     public List
      
        decode(String s) {19         List
       
         res = new ArrayList
        
         ();20         if(s == null || s.length() == 0){21             return res;22         }23         int index = 0;24         while(index < s.length()){25             int forwardInd = s.indexOf("/", index);26             int len = Integer.valueOf(s.substring(index, forwardInd));27             res.add(s.substring(forwardInd+1,forwardInd+1+len));28             index = forwardInd + 1 + len;29         }30         return res;31     }32 }33 34 // Your Codec object will be instantiated and called as such:35 // Codec codec = new Codec();36 // codec.decode(codec.encode(strs));